787 NO-BLEED SYSTEMS ARCHITECTURE
The 787 no-bleed systems architecture is shown schematically in figure 1. On the 787, bleed air is only used for engine cowl ice protection and pressurization of hydraulic reservoirs. The electrified functions are wing deicing protection, engine starting, driving the high-capacity hydraulic pumps, and powering the cabin environmental control system.
In this architecture, the power sources for the electrical system are engine-driven and auxiliary-power-unit (APU)-driven generators, while the power sources for the hydraulic system are engine-driven and electric-motor-driven hydraulic pumps. The engine-driven hydraulic power sources in the no-bleed architecture are similar to those in the traditional architecture.
In the no-bleed architecture, electrically driven compressors provide the cabin pressurization function, with fresh air brought onboard via dedicated cabin air inlets. This approach is significantly more efficient than the traditional bleed system because it avoids excessive energy extraction from engines with the associated energy waste by pre-coolers and modulating valves. There is no need to regulate down the supplied compressed air. Instead, the compressed air is produced by adjustable speed motor compressors at the required pressure without significant energy waste. That results in significant improvements in engine fuel consumption.
In the traditional architecture, the engines provide the majority of secondary airplane systems power needs in pneumatic form; in the no-bleed architecture, the engines provide the majority of airplane systems power needs in electrical form via shaft-driven generators. The traditional airplane pneumatic bleed system architecture results in less than optimum engine efficiency. Eliminating the pneumatic bleed results in a more efficient engine operation due to reduced overall airplane level power requirements — the airplane does not draw as much horsepower off the engine in cruise, so it doesn't burn as much fuel. The corresponding predicted improvement in fuel consumption, at cruise conditions, is in the range of 1 to 2 percent.
Moreover, the no-bleed architecture allows significant simplification in engine buildup due to the elimination of the pneumatic system and associated pre-coolers, control valves, and required pneumatic ducting. Figure 2 compares typical engine buildups of no-bleed engine and the traditional bleed engine.